Within flexible alternating current transmission system (FACTS) a plurality of control apparatus are known. One such FACTS apparatus is a static compensator (STATCOM). A STATCOM comprises a voltage source converter (VSC) having an ac side connected to the transmission line and a dc side connected to a temporary electric power storage means such as capacitor means. In a STATCOM the voltage magnitude output is controlled thus resulting in the compensator supplying reactive power or absorbing reactive power from the transmission line. The voltage source converter comprises at least six self-commutated semiconductor switches, each of which shunted by a reverse parallel connected diode. Since a STATCOM apparatus has no active power source it can only compensate for reactive power.
From U.S. Pat. No. 6,747,370 (Abe) a power compensation system using a converter connected to a high temperature secondary battery is previously known. The object of the compensation system is to provide an economical, high-temperature secondary battery based energy storage, which have a peak shaving function, a load leveling function and a quality stabilizing function. The known system comprises an electric power supply system, an electric load and an electric energy storage system including a high temperature secondary battery and a power conversion system. The battery is a sodium sulfur battery.
The system is arranged at an end of an electric power line. The load is a factory which under normal operating condition is provided with electric power supply from the power line. In case of power supply failure a high speed switch disconnects the power line and electric power is instead provided from the secondary battery. At the same time a back up generator is started. The known system having a sodium sulfur battery indicates that the power compensating system provides low power during a long time.
In one mode of operation the battery is providing extra energy to the factory during daytime while being recharged during night. In order to supply a factory with uninterruptible power there are arranged ten parallel connected battery units of 1280 V, each having a converter of 500 kW. In a further embodiment ten battery units are parallel connected in series with a 5 MW converter. In this embodiment a group of spare batteries is arranged for use with the high temperature battery circuit. In the event of a battery unit having a failure the failed unit is disconnected and the spare group is connected in parallel with the circuit.
Restoring power after a wide-area power outage can be difficult. A plurality of power stations needs to be brought back on-line. Normally, this is done with the help of power from the rest of the grid. In the absence of grid power, a so-called black start needs to be performed to bootstrap the power grid into operation.
To provide a black start, some power stations are typically equipped with small diesel generators which can be used to start larger generators (of several megawatts capacity), which in turn can be used to start the main power station generators. Generating plants using steam turbines require station service power of up to 10% of their capacity for boiler feedwater pumps, boiler forced-draft combustion air blowers, and for fuel preparation. It is, however, uneconomic to provide such a large standby capacity at each station, so black-start power must be provided over the electrical transmission network from other stations.
A typical sequence (based on a real scenario) might be as follows:                A battery starts a small diesel generator installed in a hydroelectric generating station.        The power from the diesel generator is used to bring the hydroelectric generating station into operation.        Key transmission lines between the hydro station and other areas are energized.        The power from the hydro dam is used to start one of the coal-fired base load plants.        The power from the base load plant is used to restart all of the other power plants in the system including the nuclear power plants.        Power is finally re-applied to the general electricity distribution network and sent to the consumers.        
To restore the power after an outage is not an easy process. Small disturbances are continually occurring while the system is weak and fragile during the restoration process, and the grid will experience different conditions from a dead network over a variety of weak network conditions to a normal strong AC network. In order to maintain the frequency and voltage stability during the restoring process, an overall coordinated system restoration plan is necessary. Hence a black start contains a first stage where the network is energized. In this stage the frequency and a phase angle must be established. In a second stage which is the recovery stage the network is unstable and very vulnerable. In this stage a certain slack must be provided by which is meant that energy must be provided to the network during part of the time while during another part of time energy must be transferred away from the network.